A dynamic spectrum access (DSA) is for the efficient use of spectrum resources. With the dynamic spectrum access, a radio communication system can correspond to a large number of users (user terminals) as compared with a conventional radio communication system and the throughput can be increased.
As the radio communication system using the dynamic spectrum access (DSA), a cognitive radio system (CRS) is known. In the cognitive radio system (CRS), an unlicensed secondary user (SU) is permitted to have dynamic access to a spectrum licensed to a primary user (PU) that can perform radio communication.
In the cognitive radio system (CRS), spectrum sensing is performed to realize the dynamic spectrum access (DSA). In addition, in the cognitive radio system, a spectrum use situation is analyzed on the basis of sensing information obtained as the result of the spectrum sensing and an accessible spectrum (white space) is determined. Thereby, the secondary user (SU) can perform communication using the white space.
Meanwhile, in the cognitive radio system, when the secondary user (SU) cannot detect a pilot signal of the primary user (PU) and starts communication with the same frequency band as a spectrum used by the primary user (PU), interference is caused. Further, when the secondary user (SU) performs communication with power greater than a predetermined limit value, the interference is caused.
A technology for setting interference restriction to the secondary user (SU) to prevent the interference from being generated in communication of the primary user (PU) is suggested (for example, see Non-Patent Document 1). According to the technology described in Non-Patent Document 1, beam forming is performed to suppress the interference.
In this case, in order to perform the beam forming, the secondary user (SU) needs to obtain momentary channel information relating to a channel from a radio transmitter of the secondary user (SU) to a radio receiver of the primary user (PU). However, in order to obtain the momentary channel information, it is needed to track a statistical characteristic of a fading channel from the secondary user (SU) to the primary user (PU) in a short period of time. For this reason, a process for obtaining necessary information is complicated. As a result, an indirect cost (overhead) of the radio communication system increases.